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EPIC 220221272
EPIC 220221272 (also designated 2MASS J01215987+0045044, TIC 399702475, UCAC4 454-001694, and SDSS DR9 1237663204923146374, informally named Dagr) is a small red dwarf star of K2 Campaign 8 within the constellation of Leo, the lion. It has one of the most expansive and diverse candidate systems ever found on Exoplanet Explorers. First noted by Vidar87 in May of 2017, it was originally thought to have 5 planets before two were found to be false positives. Eventually, one of the false positives turned out to be a real signal, while two more candidates were later discovered. Over nearly a year, multiple analyses done by various Exoplanet Explorers users continued to refine the parameters of the 5 good candidates. It was chosen in May 2018 for a collaborative study done by Planet Hunters and EE citizen scientists, where some of the candidates were validated and more potential candidates in or near the habitable zone were identified. The entire system has been informally named after Norse deities and locations. Nomenclature and History EPIC 220221272 was first catalogued by the Two-Micron All Sky Survey (2MASS) and was given the designation 2MASS J01215987+0045044 based on its Right Ascension and Declination. When observed by the Kepler Space Telescope during K2 Campaign 8, it was given the Ecliptic Plane Imput Catalogue number of 220221272. A few months after the discovery of the system, it was named after various deities from Norse mythology at the request of one of its original discoverers, Vidar87. The star was named after the Norse sun god Dagr; Candidate 03 was named after the god Vidar; 220221272 b was named for Hermodr the messenger god; 220221272 c was named after the giantess Gerth; Candidate 05 was named after the realm of fire Muspell; 220221272 d was named for the god of life and fertility Freyr; and Candidate 06 was named after Ithunn, the goddess who distributed the apples of immortality. In early May of 2017, about a month after the launch of the Exoplanet Explorers project, user Vidar87 came across three candidate planets around the star with orbital periods of 2.2, 6.6, and 13.4 days. A second EE user, shutcheon, did his own analysis and found two more: a smaller candidate orbiting every 4.2 days and a long-period candidate from a potential single transit event. The planets were all calculated to be either a little smaller than Earth to about 40% bigger, meaning they were all likely rocky. However, one of the scientists in charge of EE, Jessie Christiansen, did an analysis of the system and found the 4.2 day signal to possibly be stellar noise. The long-period transit was also found to be nothing more than a data glitch. A few days later, shutcheon did another analysis of the EPIC 220221272 system and found that the 4.2 day signal likely wasn’t noise and was instead probably a legitimate planet. A few weeks later, Vidar87 found a fifth candidate orbiting every 9.6 days. Some time after that, user ProtoJeb21 found a potential sixth candidate orbiting within the system’s conservative habitable zone every three weeks. However, he later decided this was most likely a false positive signal caused by image noise. Over the next several months from October 2017 to April 2018, he did several re-analayses of the system, using the latest updates to the program LcViewer to further refine the physical and orbital parameters of the five likely candidates. In February 2018, his analysis of the 2MASS magnitude data for EPIC 220221272 suggested that it could be slightly smaller than expected. In April 2018, he found that there actually could be a sixth planet, this time orbiting every 32 days. EPIC 220221272 is one of the systems under analysis for a joint Planet Hunters/Exoplanet Explorers study. On May 29th, 2018, the first initial results were posted on Planet Hunters. Using the program VESPA, it was revealed that the 4.2, 6.6, and 13.4 day signals were highly likely to be actual planets; their false positive probabilities are low enough (less than 0.001%) for them to be considered validated. However, VESPA found some problems with the 2.2 and 9.6 day signals, and more analysis is being done to determine their true nature. While PH user ajamyajax ran more VESPA analyses on the remaining two good candidates, ProtoJeb21 searched through Exoplanet Explorers analyses to look for evidence of any additional candidates. He found four potential extra signals to investigate and determined that three were good enough for follow-up analysis, including the 21-day signal from almost a year earlier. Planetary System EPIC 220221272 has a system of multiple planets. As of July 12th 2018, there are six total candidates, five of which are validated and one requiring more analysis to confirm its true nature. It is currently tied as the largest largest candidate system ever found on Exoplanet Explorers, along with EE-1/K2-138 and K2-149 (EPIC 220522664) with 6 planets each. All of the six candidates are extremely tightly packed, all orbiting in three weeks or less. Signs of candidates beyond Candidate 01/Planet b were seen for almost a year, and as of June 3rd there are three potential signals taken from this region. Four of the inner five good candidates have radii not too much larger or smaller than that of Earth, meaning they are likely rocky in nature. The fifth planet, however, is above the 1.6 R⊕ boundary where most planets begin to transition from rocky worlds to water and/or gas-rich worlds. EPIC 220221272 d could either be a Mini-Neptune, or an ocean world with somewhere between 10% and 50% of its mass comprised of water. The sixth, seventh, and eighth candidates are similar in size to the innermost four, although candidates 06 and 08 could be smaller at around three-quarters the size of Earth. Due to their weak signals, there radii are difficult to determine as of now. Most of the planets in the system are too hot to support life in any way. EPIC 220221272 d was originally considered to be in the optimistic habitable zone, with its equilibrium temperature calculated to be about 298 K. However, data from the Gaia spacecraft have determined that EPIC 220221272 is slightly more luminous than originally thought, meaning that the fifth candidate is slightly too hot to be in the habitable zone. Even with that, it could still retain liquid water, as its stellar flux and potential composition suggest it may have a very reflective water vapor cloud layer, cooling the surface to more comfortable temperatures. The original five candidates - C03, C04, C02, C05, and C01 - have been statistically validated as real planets, with their FPPs (False Positive Probabilities) all being less than 1%. The least likely, C03/EPIC 220221272 e, has just a 0.686% chance of being a false postive; the most likely, C01/EPIC 2202221272 b, has a FPP of 0.000017%. Their radii were calculated with larger size estimates for the star, but newer parameters provided by TESS are going to be used now instead. New radii for these planets have yet to be calculated, although the radius of C06 was determined with these parameters. On June 2nd, 2018, three additional candidates beyond EPIC 220221272 b were reported, based on two analyses by ProtoJeb21 and one by citizen scientist Hans Martin. One of them is the original EPIC 220221272.06, which takes three weeks to orbit and is near a 3:2 resonance with EPIC 220221272 b. The next, designated EPIC 220221272.08 (based on the order of its discovery), was reported in April 2018 by Hans Martin in a post on Exoplanet Explorers. It takes 25.72 days to complete one orbit in an almost exact 5:4 resonance with 220221272.06 and 220221272.07, which orbits every 31.79 days. As of July 16th, only C06 is considered a good candidate; C08 was determined to be a false positive, and C07 is even more dubious than C06. The six candidates have equilibrium temperatures of 533, 432, 370, 327, 292, and 254 K, and receive 19.116, 8.26, 4.44, 2.69, 1.72, and 0.98 times as much sunlight as Earth. EPIC 220221272.06 is well within the conservative habitable zone, while EPIC 220221272 b may be at least a temperate water-rich planet. Other HZ candidates beyond C06 might exist. Category:Red dwarf systems Category:Stars